1. Field of the Invention
The present invention relates to an improvement for a pneumatically controlled spring brake of the type utilized in commercial trucks and trailers.
2. Description of the Prior Art
Pneumatic spring brakes are widely utilized in the trucking industry. A typical conventional pneumatic spring brake has both a service brake portion for slowing and stopping a truck or trailer moving under normal conditions by means of a foot valve, and also an emergency parking brake portion. One such brake is manufactured for commercial use as the Anchorlok model 3030L combination service/spring brake by Lear Siegler, Inc., Truck Products Division located in Compton, Calif. The service brake portion of this device applies the brakes of the wheels of a truck or trailer on the highway using pneumatic pressure to act against the service brake diaphragm. Air under pressure is utilized to activate the service braking action.
In a typical pneumatic brake of this type, air is maintained in a pneumatic reservoir at a pressure on the order of one hundred pounds per square inch to provide three thousand pounds of thrust against the thirty square inch effective surface area of the service brake diaphragm. Pneumatic pressure is maintained by an onboard air compressor. However, if a failure develops in the pneumatic air supply system, the service brake will not function. Therefore, an emergency brake is also provided.
In a double diaphragm spring brake the emergency or parking brake is mounted directly behind the service brake. The actuation force applied from the emergency or parking brake is applied to the brake mechanism on the vehicle wheel through the service brake. Application of the emergency brake is entirely mechanical. The actuating force by the emergency or parking brake is applied by a heavy coil spring in a spring chamber. This spring is maintained in check by air pressure acting on the opposite side of a diaphragm in the brake housing. The heavy spring within the spring brake portion of the device applies the brake on a wheel to which it is connected when there is a loss of air pressure in the spring chamber of the emergency and parking brake. Such a loss of air pressure can occur either through a break or failure in the air line applying pressure in the spring brake chamber, or by intentional exhaustion of air from the parking brake chamber, as when the truck or trailer is parked.
When pneumatic pressure is available for the spring brake, as during normal operation of the vehicle, the pneumatic pressure acts against the spring brake diaphragm to compress a heavy coil spring within the spring brake housing. This prevents the coil spring from acting against a push rod to apply the brake to the vehicle wheel. However, a pressure failure in the pneumatic line will remove the counter-balancing pneumatic force from the spring brake diaphragm and allow the heavy, compressed coil spring to act upon the push rod to apply the brake and prevent the wheel associated therewith from turning. Accordingly, the spring brake is activated both during a failure of pneumatic pressure, in which case the spring brake serves as an emergency brake, and also when the vehicle is parked, in which case the spring brake serves as a parking brake.
When compressed within the separable brake housing portions which are typically joined together by semi-circular clamping rings, the spring contains a tremendous amount of potential energy. The coil spring is quite heavy and typically weighs three pounds or more. The spring is compressed to a linear length of less than three inches from an original uncompressed length in an extended condition of from nine to twelve inches. Consequently, the compressed spring in the pneumatic spring brake housing can be extremely dangerous, especially to persons who are unaware of the danger or are unfamiliar with the structure and manner of interaction of the component parts of the spring brake. The spring is held in a compressed condition so as to exert a force on the housing of from two thousand to three thousand pounds. If an unskilled person attempts to take the spring brake apart for repair, the spring will literally explode from the housing and cause profound bodily injury, and even death.
One approach toward reducing the potential for injury when a spring brake is disassembled is to "cage" the spring prior to disassembly so that the spring exerts no pressure against the spring brake diaphragm. Caging is performed by inserting a special bolt through an access hole in the end of the spring housing, concentrically through the coils of the spring, and into a pressure plate. The bolt is equipped with a transverse cross pin at the end which is inserted through the pressure plate. The pressure plate includes a slot to admit the cross pin therethrough. Once the cross pin clears the pressure plate the bolt is turned so that the cross pin is brought into contact with a recess in the underside of the pressure plate. A nut is then threadably engaged on the threaded end of the bolt which protrudes from the access hole. The nut is tightened down against the housing adjacent to the access hole. Tightening of the nut at the exposed end of the bolt draws the pressure plate toward the end of the housing in which the access hole is formed, thereby retracting the spring pressure plate and the coiled spring away from the diaphragm.
Caging of the spring can be quite dangerous however if the pressure plate has corroded or the spring has broken. Furthermore, it is not uncommon for springs to break, due to fatigue, especially when a plug has fallen from the access hole and corrosion has accelerated fatigue. Consequently, the disassembly of a spring brake still entails considerable risk of bodily injury and damage.
In conventional spring brakes, the clamping ring which joins separable housing portions of the spring brake is constructed of two generally semi-circular clamping ring elements having radially outwardly turned attachment ears which are bolted together. Each of the housing portions is formed with a radially outwardly directed lip or rim. These outwardly directed lips or rims are pressed toward each other throughout their perimeters. When nuts are tightened on the bolts the distance of separation between the juxtaposed attachment ears is reduced as the bolt assemblies draw the clamp ring elements toward each other to firmly grip and secure the separable housing portions of the brake together. The lips or rims of two adjacent housing portions are entrapped and held together by a clamping ring. The clamping ring encircles the housing portions at their mutual interface. However, any person with the most rudimentary of workshop tools can unthread the nuts from the bolts and thereby inadvertently release the tremendous power of the coil spring.